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Class B Noxious Weed

Purple loosestrife drawing

History

Purple loosestrife is native to Europe and was introduced to the United States as an ornamental garden flower. With no natural enemies here, loosestrife is very prolific and invades wetland and shoreline areas, crowding out native plants and reducing habitat diversity. No wildlife species are known to use this plant, and the dense thickets of dead stems deter waterfowl and other wildlife access. It is now illegal to buy or sell purple loosestrife in Washington State.

Method of spread

Purple loosestrife grows in all counties in our area. It is highly invasive and spreads by seed, runners, and stem cuttings. Its seed production (up to 3,000,000 seeds per plant stalk), and an extensive root system make it quite competitive.

Methods of control

Purple loosestrife is controlled using herbicide and mechanical methods. Flower tops can be clipped and the stalks dabbed with herbicide. Hand removal methods which include digging up the plant can be effective for small areas. Biological control agents have also been used with some success.

Identification

  • commonly confused with fireweed and spirea
  • frequently occurs in very dense and colorful populations
  • leaves are two to four inches long, the upper ones smaller, and are lance-like with heart-shaped bases
  • leaves are typically opposite (in pairs along the stem), although they are sometimes alternate or whorled (in threes or spiraled)
  • stems are up to six feet tall, and unbranched stems are woody and distinctly four-sided
  • very showy flowers appear in dense, elongate spikes with the magenta petals blooms July to September

More about purple loosestrife

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For questions about lakes in King County, please contact lakes@kingcounty.gov or call the Water and Land Resources Division front desk at 206-477-4800.